1. Field of the Invention
The present invention relates to the production of commercial quicklime and its end products and specifically to a process for controlling the presence of soluble sulfate ions during the slaking of quicklime which would otherwise lead to undesirable agglomeration of the fine calcium hydroxide particles produced.
2. Description of the Prior Art
Lime, in its different forms, has a variety of uses. It is commonly used in treating waste water and sewage. It is used in agriculture to neutralize acidic soils and to provide nutrients for sustaining plant life. Lime is also used extensively in construction for the stabilization of soils and as a component in a variety of building materials. These are but a few of the many uses of this versatile material.
The general term “lime” is often used interchangeably to mean both quicklime (calcium oxide) and hydrated lime (calcium hydroxide). Quicklime is produced by heating limestone (calcium carbonate) in a kiln at extreme temperatures to “calcine” the material and thereby drive off carbon dioxide. Quicklime is usually in the form of lumps or pebbles. In order to further process lime and improve the ease with which it is handled, quicklime is often contacted or mixed with water. The water reacts with the quicklime in an exothermic reaction to form hydrated lime. This is often referred to as “slaking.” During the slaking of quicklime, large amounts of heat are given off which can significantly raise the temperature of the slurry. Water can then be driven off to produce dry, hydrated lime which is usually a powder. Technically, the terms “hydration” and “slaking” are synonymous and interchangeable. However, according to popular usage of these terms, hydration yields a dry powdered hydrate, whereas slaking involves more water, producing wet hydrates, sometimes referred to as putties, slurries, milk of lime and lime water, depending upon the amount of excess water they contain.
It is well established that sulfur in the form of sulfates is a undesired impurity in commercial quicklime. For example, the sulfur is detrimental for the use of quicklime in the steel industry because one of its applications is to remove sulfur during the flux operation of purifying iron into steel. The presence of sulfur in any form is detrimental in this market.
In other markets where quicklime is slaked to produce a milk of lime or lime slurry, the presence of sulfate ions in the quicklime causes an agglomeration reaction during the slaking process which causes the fine particles of calcium hydroxide to stick together and thus settle out of suspension.
In some instances in the past, sulfate ions have actually been intentionally introduced into the lime slaking operation. For example, well-established technology exists which involves the addition of gypsum or sulfate ions to quicklime during the slaking operation to increase the solids content of lime slurry in a controlled manner. U.S. Pat. No. 4,464,353, issued Aug. 7, 1984, to Norman L. Hains teaches that, in the production of a lime slurry, the timely addition of sulfate compounds, preferably calcium sulfate, to the aqueous slaking medium prior to the introduction of calcium oxide (quicklime) retards the chemical reaction of the calcium oxide with the aqueous slaking medium, thereby forming a lime slurry having decreased solubility and increased particle agglomeration. According to the teaching of that patent, the described process affects the physical properties of the lime slurry formed by allowing the formation of larger crystals of calcium hydroxide, thus increasing the average particle size by agglomeration.
Despite the advantages obtained through the addition of sulfate ions during the slaking operation under the controlled conditions described above, it is known that the presence of excess sulfate ions will cause an unstable lime slurry which will settle out in storage tanks and in transport vehicles.
In many instances, it is therefore desirable to limit the presence or availability of free sulfate ions during the quicklime slaking operation.
An opposing consideration for the lime manufacturer, however, is the fact that there is an advantage in the production of quicklime to increase the sulfur content in the product. This results from the fact that the higher sulfur content fuels used in the step of calcining the limestone to form quicklime are less expensive then lower sulfur content fuels. Thus, the manufacturers of quicklime would like to use as high a sulfur fuel as possible, balancing the sulfur content in the quicklime and operational conditions in the kiln.
The quality and type of fuel exert a dramatic effect on the quality of lime produced. The major fuel sources at the present time include solid fuels, such as bituminous coal, anthracite coal, coke and producer gas, natural gas and fuel oil. While sulfur exists in limestone homogeneously as calcium sulfate or heterogeneously in the mineral pyrite in amounts of about 0.01 to 0.12%, the calcining fuel generally introduces more sulfur into the calcination process than does the limestone feed, natural gases being the exception. For example, coal used for lime manufacture typically contains 0.5-3.5% and fuel oils contain nearly as much.
There exists a need, therefore, for a process which would allow the use of higher sulfur content fuels in the step of calcining the limestone to form quicklime which would, at the same time, control the presence of soluble sulfate ions during the slaking of quicklime which would otherwise lead to undesirable agglomeration of the calcium hydroxide particles produced.
A need also exists for such a process which would allow the use of solid fuel sources in the calciner, such as coal, rather than requiring the use of more expensive natural gas as a fuel source.
A need exists for such a process which would be easily implemented as a part of the slaking operation without requiring drastic changes in operational procedures or equipment.